[PATCH v3 1/5] x86,fs/resctrl: Remove unused non-linear bandwidth delay mapping

[Reinette Chatre]

Intel MBA throttles memory bandwidth with delay values, while resctrl
exposes memory bandwidth to user space as a percentage. When delay values
are linear there is a direct mapping between the two. Support for
non-linear delay values would need a pre-calibrated table mapping
percentages to delay values.

Commit

  05b93417ce5b ("x86/intel_rdt/mba: Add primary support for Memory Bandwidth Allocation (MBA)")

added a placeholder for such a table to the resctrl_membw structure, but
the table was never populated and never read. rdt_get_mb_table(), which
would provide it, always fails. As a result, an Intel MBA resource with
non-linear delay values is never marked as capable of allocation, and its
control values are never written to hardware.

Remove the unused mapping table and fold the linear percentage-to-delay
conversion into the function that writes the control values to hardware.

Keep the check that delay values are linear as a defensive measure in
preparation for supporting multiple controls, which turns the memory
bandwidth control into a generic scalar control that indicates whether it
requires linear values.

Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
Changes since RFC v1:
- Also drop separate delay_bw_map() and instead integrate the only
  mapping to when control value is written.

Changes since RFC v2:
- Rework changelog.

Changes since v1:
- Rewrite changelog, fix subject prefix.
- Match error message about linear support with similar message during
  enumeration added in next patch.
- Add Chenyu's Reviewed-by tag

Changes since v2:
- Add Jinjie and Tony's Reviewed-by tags.
---
 arch/x86/kernel/cpu/resctrl/core.c | 23 +++++++----------------
 include/linux/resctrl.h            |  2 --
 2 files changed, 7 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index 55214d6fdc49..f9677e52790e 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -323,29 +323,20 @@ static void mba_wrmsr_amd(struct msr_param *m)
 		wrmsrq(hw_res->msr_base + i, hw_dom->ctrl_val[i]);
 }
 
-/*
- * Map the memory b/w percentage value to delay values
- * that can be written to QOS_MSRs.
- * There are currently no SKUs which support non linear delay values.
- */
-static u32 delay_bw_map(unsigned long bw, struct rdt_resource *r)
-{
-	if (r->membw.delay_linear)
-		return MAX_MBA_BW - bw;
-
-	pr_warn_once("Non Linear delay-bw map not supported but queried\n");
-	return MAX_MBA_BW;
-}
-
 static void mba_wrmsr_intel(struct msr_param *m)
 {
 	struct rdt_hw_ctrl_domain *hw_dom = resctrl_to_arch_ctrl_dom(m->dom);
 	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(m->res);
 	unsigned int i;
 
-	/*  Write the delay values for mba. */
+	if (!m->res->membw.delay_linear) {
+		pr_warn_once("Non-linear MBA delay values not supported\n");
+		return;
+	}
+
+	/* Program bandwidth percentage mapped to linear delay value. */
 	for (i = m->low; i < m->high; i++)
-		wrmsrq(hw_res->msr_base + i, delay_bw_map(hw_dom->ctrl_val[i], m->res));
+		wrmsrq(hw_res->msr_base + i, MAX_MBA_BW - hw_dom->ctrl_val[i]);
 }
 
 static void cat_wrmsr(struct msr_param *m)
diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 10dfdca7f4bf..b63dc62690ca 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -253,7 +253,6 @@ enum membw_throttle_mode {
  * @throttle_mode:	Bandwidth throttling mode when threads request
  *			different memory bandwidths
  * @mba_sc:		True if MBA software controller(mba_sc) is enabled
- * @mb_map:		Mapping of memory B/W percentage to memory B/W delay
  */
 struct resctrl_membw {
 	u32				min_bw;
@@ -263,7 +262,6 @@ struct resctrl_membw {
 	bool				arch_needs_linear;
 	enum membw_throttle_mode	throttle_mode;
 	bool				mba_sc;
-	u32				*mb_map;
 };
 
 struct resctrl_schema;
-- 
2.56.0